This invention relates to an article coated with an environmentally resistant coating and to a method for making such an article. More particularly, it relates to articles made of a high temperature alloy including a protective coating including Al.
It has been a practice in apparatus for use in power generation, for example gas turbine engines, to provide certain components designed to operate at elevated temperatures in an oxidizing atmosphere with environmental resistant or protective coatings. Such coatings are intended to resist attack on the article substrate alloy, during service operation, by such a strenuous atmosphere and/or its contaminants. For example, turbine engine components designed for use in an engine portion within or downstream of a combustion section and made of a high temperature superalloy have been provided with an environmental resistant coating that includes the element Al. Reported forms of such coatings include diffusion aluminide coatings the method of application of which includes processing using Al in slurry, in a vapor, or carried by an ion, for example a halide ion carrier. The method is conducted at an elevated temperature sufficient to enable significant interdiffusion of an outer layer including Al with the substrate. As has been widely describe in the art, the result of such significant interdiffusion is a diffusion portion or zone, between the outer or additive layer or zone and the substrate, of a thickness generally about the same as that of the additive layer.
Certain components, for example gas turbine engine turbine blades, vanes and struts, are designed to operate at a temperature greater than that at which desired mechanical properties of the substrate or structural alloy can be maintained. Therefore, it has been a practice to cool such articles with a fluid such as air flowing within and/or about the article. Forms of fluid or air-cooled gas turbine engine airfoils are described in such U.S. Pat. Nos. as 5,458,461 and 5,503,527xe2x80x94Lee et al. (patented, respectively, on Oct. 17, 1995 and Apr. 2, 1996).
When an environmental resistant diffusion aluminide coating is applied, according to current practice, to an outer wall of such an air-cooled airfoil, the relatively thick diffusion zone involves or becomes part of a portion of the outer wall. In some instances during service operation of such an article, degradation of the environmental resistant diffusion aluminide has been observed and repair is required before the article can be returned to service. Some repair methods comprise removal of the entire coating, including the outer or additive portion as well as the diffusion zone, prior to or in preparation of application of a repair coating. Removal of the coating diffusion zone, that has involved a part of the wall, necessarily results in significant reduction in the article wall thickness, and the consequent reduction in the further repair capability of a re-coated article.
The present invention, in one form, provides a method for applying to an article substrate an aluminide coating which develops, as-deposited, a relatively thin diffusion zone between an article substrate and a coating outer portion, sometimes called the additive zone. For example, in one form, the as-deposited diffusion zone is of a thickness of less than about 1 mil or no greater than about 60% of the coating outer portion or additive zone, as measured by chemical analysis of the coated article. This method minimizes involvement of a wall of the article. The aluminiding method combines the use of a relatively low aluminiding temperature below that at which significant interdiffusion occurs between an outer layer including Al and the substrate, with the use of a relatively high Al activity Al source material. In one example, the combination includes an aluminiding temperature in the range of about 1650-1800xc2x0 F. and a source material including Al at least at about 40 wt. %.
One embodiment of the article of the present invention includes a metallic substrate, for example a high temperature superalloy, and an aluminide additive or outer portion bonded with the substrate through a diffusion zone, between the outer portion and the substrate, of an as-deposited thickness of less than about 1 mil or no greater than about 60% of the outer portion or additive zone.